In a typical refrigeration system, the compressor operates within a range of acceptable pressure ratios. The pressure ratio is defined as the ratio of compressor discharge pressure to suction pressure. In a compressor operating in normal mode, i.e., with no capacity unloading devices activated, the relationship between discharge pressure and suction pressure is linear. The maximum pressure ratio is determined by the compressor manufacturer. Operating the compressor above the maximum pressure ratio can cause damage to the compressor and other system components, or may cause a safety shutdown resulting in a total loss of cooling provided by the system.
Therefore, what is needed is a control algorithm that can determine when a compressor in a refrigeration system is operating at or above its maximum pressure ratio, and can promptly unload the compressor to avoid damage to the system, without shutting down the system. What is further needed is a control algorithm to unload a compressor that has reached a maximum pressure ratio, while continuing operation of the compressor and then to reload the compressor to respond to capacity demand only after the pressure ratio has stabilized.